Have you seen the Disney movie, Black Hole, from 1979? It was typical Disney fare; clueless good guy, camouflaged bad guy, comic relief characters… And all the action happened on a ship that sat in space, just outside the ‘point of no return’ of a Black Hole, which was depicted as a whirlpool of light.
Even then, I knew a black hole was ‘a region of space having a gravitational field so intense that no matter or radiation can escape.’ So that depiction of a black hole bothered me. If light can’t escape, then the black hole would not appear as a whirlpool of light. It should be black. Even light from stars beyond the black hole shouldn’t be seen; it would be bent and swallowed before it reached the observer. Right?
That’s what I was thinking, anyway.
These days, the idea of a Black Hole is not quite so… black and white.
NASA says a black hole is a place in space where the gravity is so intense, light cannot escape. In the same article, they say that if a black hole is located close to a star, high-energy (invisible) light is released. I suppose this is a special case, but I think they should have started by saying no visible light can escape.
Another NASA webpage states that black holes cannot be seen, because (visible) light cannot escape. But anything in the vicinity is effected. Dust bits will fall into the black hole, getting closer and closer until they hit that ‘point of no return’, when the light reflecting off them can no longer escape. The gravity will tug at any stars and planets in the area, making them wobble as they try to resist. Stars might even be pulled apart, and slurped up. As the star matter accelerates toward the hole, it emits x-rays, which can be detected by the proper equipment.
But something can and does escape from black holes, in a way. Some matter that is falling into a black hole ricochets off the event horizon (point of no return) instead of going through it. It bounces away at a speed so great, the jet of material can be detected relatively easily.
If that’s not enough for you, then consider a super-black hole that spins really, really fast. This spin creates a charge-separated magnetosphere, which forms parallel electrical charges around the poles. Particles accelerate to very close almost the speed of light and are thrown out into space as gamma radiation bursts. Bursts have been observed from the massive black hole at the center of Galaxy IC310. However, the description of how they are created is just a theory, since no one can see inside the event horizon to see what’s going on.
So, Black Holes are pretty black, unless something is escaping.